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Plenary sessions fall under three broad topics:

Each plenary session will pair a series of related talks with facilitated, active discussion. This interactive format is intended to foster both broader and deeper collaboration and understanding amongst our community, to propel progress and enhance success by breaking down silos both within the high-energy solar physics and among solar and space physics in general.

There are three invited talks associated with the three broad topics (see abstracts):

  • Nicki Viall – Energy storage and release in the solar atmosphere
  • Jim Drake – Particle heating and acceleration during magnetic reconnection
  • Joel Allred – Thermal and non-thermal transport of energy in solar flares

Finally, there are four special topics (see below).  These breakout sessions are intended to allow for deep conversations on particular topics.

  • Magnetic shear, reconnection, and particle acceleration
  • Future of observing ion acceleration
  • Electron number problem: Reconciling X-ray and microwave observations
  • Synergy of multiple particle-acceleration mechanisms

Topic 1: Energy storage and release – e.g., reconnection, eruption initiation

Leaders: Kathy Reeves (CfA), Ryan French (NSO)

 

Solar Eruptive Events (SEEs), of which flares are a primary component, are understood to be powered by the release of magnetic energy stored in the solar corona. While reconnection is thought to play an important role in flare energy release, its role in the initiation of eruptive events and its evolution during those events remains a topic of intense study. An additional aspect relevant to the energy release during eruptions is how the local and global environments affect these events. In this session, we aim will focus on the following critical questions:

  • What observable parameters (e.g., magnetic configuration) influence the energy partition of solar eruptive events (e.g. flare vs. CME, thermal vs. nonthermal)?
  • How do energy release mechanisms evolve through the impulsive/gradual phases of an eruptive flare?
  • How do observations constrain our understanding of reconnection energy release in flares (e.g., three-dimensional structure, turbulence, plasmoids, guide field)?
  • How do local and global environments influence eruptive events?  Are there observable pre-eruption signatures, both globally and locally?
  • What recent or future observations can be used to test competing models for eruption initiation and fast energy release?

 

Topic 2: Energy conversion – e.g., particle acceleration (electrons and ions) and plasma heating

Leaders: Xiaocan Li (Dartmouth), Jessie Duncan (GSFC)

 

During solar flares and solar eruptions, magnetic energy is converted into plasma kinetic energy, heating the coronal plasma to millions of degrees and/or accelerating a large number of particles (electrons, protons, and heavy ions) to high energies. The hot plasmas and high-energy particles emit radiation across all energy bands. Some energetic particles can be released into interplanetary space and become impulsive SEP events observed in situ. Despite decades of research, many critical questions regarding particle acceleration and plasma heating remain unanswered. 

 

  • When and how are particles accelerated during solar eruptions?
  • What are the connections between plasma heating and particle acceleration in solar eruptions?
  • What is the dominant mechanism for converting magnetic energy into plasma heating in quiescent active regions and/or the quiet corona?
  • What insights can we gain from other fields (e.g., magnetosphere and solar wind) about plasma heating and particle acceleration?

 

This session aims to discuss these high-level fundamental questions and consider what new insights, observations, and models are needed to make progress toward definitive answers. Contributions from the broad heliosphere community (e.g., solar wind and magnetosphere) are also welcome.

Topic 3: Energy transport – probing high energy solar phenomena using remote sensing and in situ diagnostics

Leaders: Pascal Saint-Hilaire (SSL) , Sijie Yu (NJIT)

 

Solar energetic events, such as flares and coronal mass ejections (CMEs), are primary sources of suprathermal particles associated with energetic particle occurrences. Despite this, crucial questions about particle source regions and the physical mechanisms driving acceleration and transport within and beyond the corona remain unresolved. Thoroughly understanding these processes necessitates an analysis of energetic particle properties at their acceleration sites and during propagation throughout the heliosphere.

This session aims to explore and constrain the driving forces behind high-energy events by integrating remote sensing and in situ measurements of energetic particles. Coordinated observations from heliospheric observatories will be employed to examine particle transport, evolution, and longitudinal extent across the heliosphere. Additionally, this session will investigate the origins of seed populations and the conditions in acceleration regions using chemical abundances, ionization states, and energetic particle spectra.

  • Transport effects on flare-accelerated electrons in and around the flare sites: collisions, scattering, trapping, wave-particle interactions, and other collisionless mechanisms.
  • Escape of flare-accelerated electrons from solar eruption regions to interplanetary space, and their association with magnetic topology. Comparison of flare-/CME-accelerated electron distribution from in-situ and remote sensing techniques.
  • Investigating the evolution of accelerated ion populations throughout the heliosphere (e.g., solar and in-situ populations, different species, He-3/He-4)
  • Assessment of observation capabilities for past, current, and needed in-situ and remote-sensing instrumentation.

Invited Talks

Nicki Viall – Energy storage and release in the solar atmosphere

There have been remote observations of the solar atmosphere for centuries, and in situ measurements of the heliosphere for almost 60 years. Computer simulation capabilities have vastly improved, and simulation techniques of the coupling between the layers of the sun, through the solar atmosphere, and out into the heliosphere continue to advance. Yet there are longstanding, major unsolved questions of how the solar atmosphere is energized on small and large scales, as well as how the solar wind and heliosphere are formed. The answers involve universal physical processes of energy storage and release, manifested through magnetic reconnection, turbulence, and waves. These questions remain unanswered because observations and simulations are limited to narrow aspects of the physics and/or system, and thus cannot capture cross-scale and cross-region coupling. We compare different forms of energy storage and release in the solar atmosphere and describe progress that could be made with new observations and simulation capabilities that link the kinetic scales, through the mesoscales, to the global processes.

Jim Drake – Particle heating and acceleration during magnetic reconnection

How the magnetic energy released during reconnection is transferred to hot electrons and ions and nonthermal components is a topic of broad importance both in the heliosphere and the broader universe. I will review observations and our current understanding of the mechanisms that drive both heating and nonthermal particle production with an emphasis on non-relativistic reconnection. An organizing parameter is the magnetic energy released per particle WB = mi VA2. In regions where WB is large such as in the Earth’s magnetotail, solar flares and the solar wind near the sun, observations reveal both hot thermal and nonthermal, powerlaw components. Single x-line models fail to explain the generation of the nonthermal component. However, simulations reveal that reconnection becomes turbulent in the high WB environment. Magnetic energy release and particle acceleration therefore take place in a multi-x-line environment. A major surprise is that the energy gain of the most energetic particles is dominated by Fermi reflection in growing and merging magnetic flux ropes rather than the parallel electric fields in kinetic scale boundary layers. On the other hand, the large-scale parallel electric potential that develops to maintain charge neutrality appears to control the heating of the hot thermal electrons during reconnection. The implication is that the kinetic scale boundary layers that control the parallel electric field are not important in energy release in large-scale systems. Particle-in-cell simulations are beginning to reveal powerlaw distributions of both electrons and protons. However, the PIC models fail to produce the extended powerlaws seen in flare observations because of inadequate separation of kinetic from macroscales. A new computational model, kglobal, has been developed that blends MHD dynamics with electron and ion particles but eliminates all kinetic scales. Simulations of reconnection in a macro-scale system reveal powerlaw distributions of electrons and protons that extend nearly three decades in energy and that the dominant control parameter is the ambient guide magnetic field. The results suggest that modeling of particle acceleration in realistic macro-scale geometry will be possible.

Joel Allred – Thermal and non-thermal transport of energy in solar flares

Flares are characterized by the impulsive release of magnetic energy, which is rapidly transported throughout the flare arcade driving fast flows and intense brightenings. Much of this energy is transported via non-thermal particles that are accelerated during the magnetic reconnection process. Non-thermal electrons heat the flaring chromosphere producing fast evaporation into the corona. Non-thermal ions are also likely accelerated and may play a key role in producing the acoustic pulses deep in the solar atmosphere known as sunquakes. Our current models of flares are able to accurately reproduce many key observables, including the speed of chromospheric evaporation flows, plasma densities and atomic line intensities. However, after the cessation of impulsive heating, the models predict time scales for slowing flows and cooling to quiescence that are an order of magnitude faster than observed. Such a discrepancy indicates missing ingredients in the models. As shown in recent work, turbulence suppresses thermal conduction and is a likely candidate for explaining long duration cooling times. Here we describe recent work (1) modeling the turbulent suppression of thermal conduction and its role in energy transport in flares and (2) modeling energy transport via non-thermal ions and their effects on the solar atmosphere with particular attention to their ability to produce sunquakes.

 

Special topics

Magnetic shear and reconnection

Leaders: Jiong Qiu and Joel Dahlin

The future of observing ion acceleration

Leaders: Albert Shih and Melissa Pesce-Rollins

The accelerated-electron number problem and reconciling X-ray and microwave observations

Leaders: Brian Dennis et al.

Synergy of multiple particle-acceleration mechanisms

Leaders Xiaocan Li and Kathy Reeves